In medical diagnostic radiography the object is to obtain a viewable silver image from which a medical diagnosis can be made while exposing the patient to a minimal dose of X-radiation. Patient exposure to X-radiation is minimized by employing a dual-coated radiographic element in combination with front and back fluorescent intensifying screens. A portion of the X-radiation transmitted through the patient's anatomy is absorbed by each of the front and back intensifying screens. Each screen emits light in response to X-radiation exposure, and the emitted light from the front and back screens imagewise exposes the front and back emulsion layers of the dual-coated radiographic element. With this arrangement, patient exposure to X-radiation can be reduced to about 5 percent of the X-radiation exposure level that would be required for comparable imaging using a single emulsion layer and no intensifying screen.
Unlike photographic images, which arc taken in small formats and then enlarged for viewing, radiographic images are normally viewed without enlargement. Thus, very large formats by photographic standards are required. Further, unlike color photography, wherein silver is reclaimed in processing, the silver in radiographic elements is often not reclaimed for years, since the images are required to be available to substantiate diagnoses. Further, usually a number of images are obtained when subject matter of pathological interest is observed. Thus, there has been in medical diagnostic imaging a long standing need to minimize to the extent feasible the silver contained in the elements.
Although higher covering power had been previously attributed to tabular grain emulsions, Dickerson U.S. Pat. No. 4,414,304 recognized that tabular grain emulsions having average tabular grain thicknesses of less than 0.2 .mu.m are capable of providing higher covering power than thicker tabular grain emulsions. With this discovery it would seem logical to incorporate the thinnest possible tabular grains in radiographic elements, but this has not occurred in practice. For example, ultrathin tabular grain emulsions, those with mean tabular grain thicknesses of less than 0.07 .mu.m, are preferred for many photographic applications, but such emulsions are rarely, if ever, employed in radiographic elements.
What has stifled silver coating coverage reductions by employing tabular grains of minimum mean thicknesses is the observation that image tone becomes increasingly warm as mean tabular grain thickness is decreased. Radiologists strongly prefer images that have a "cold" (e.g., blue-black) appearance as compared to those with a "warm" (e.g., brown-black) appearance. Typically radiographic elements employ blue-tinted supports in combination with silver halide emulsions selected to provide overall colder image tones.
Attempts to achieve both higher covering power and colder image tones in thin tabular grain emulsions have been diligently pursued without success. For example, Hershey et al U.S. Pat. No. 5,292,631 discloses alkylthio-substituted azoles to increase the covering power of high bromide tabular grain emulsions. However, alkylthio-substituted azoles are reported by Hershey et al U.S. Pat. No. 5,292,627 to produce colder image tones in only nontabular grain emulsions with mean ECD's of less than 0.3 .mu.m.